Field of the Invention
The present invention relates to wireless communications and, more specifically but not exclusively, to channel estimation in wireless communications systems employing macrocells having large-scale antenna system (LSAS also known as massive MIMO (multiple in, multiple out)) base stations and small cells having small-cell base stations like relays, repeaters, and full-fledged small-cell base stations.
Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
In conventional cellular communication systems having mobile devices that communicate wirelessly with base stations, each mobile device repeatedly transmits an uplink pilot signal based on a known, (absolutely or relatively) unique pilot sequence to its associated base station (BS). For each mobile unit, the base station receives the corresponding transmitted uplink pilot signal and estimates a different set of instantaneous (i.e., current) channel state information (CSI) data for the corresponding transmission channel between each mobile device antenna and each base station antenna. In a conventional cellular communication system, a base station characterizes the instantaneous CSI data for each of its communication channels in each time slot having a duration short enough to ensure that the mobile device does not move more than a quarter-wavelength of the channel's communication frequency.
To meet the ever-rising demands of wireless communications, cellular operators are deploying cellular communications systems that combine large-scale antenna system (LSAS) base stations and small-cell base stations, where each LSAS base station (BS) has tens or hundreds of antennas and is associated with a number of different small-cell BSs, and each small-cell BS communicates with a relatively small number of wireless users (e.g., mobile devices). Operating with a large ratio for the number of LSAS BS antennas to the total number of small-cell BS antennas under simultaneous service can yield large increases in both spectral efficiency and energy efficiency. As the number of antennas increases and power is commensurately reduced, conjugate beamforming on the forward link (i.e., downlink) and matched-filtering on the reverse link (i.e., uplink) asymptotically approach near-optimal performance.
In these systems, each small-cell BS functions as a wireless node communicating backhaul data wirelessly with its associated LSAS BS. Unfortunately, as the number of antennas increases, the processing load required to estimate the CSI data for each wireless backhaul channel using conventional techniques becomes prohibitively expensive.